Gettering processes for electric lamps and similar devices

ABSTRACT

In the process of gettering, the rate and extent (to about 100 percent) of thermal decomposition of P3N5 getter are increased by providing a catalyzer metal in contact with the solid P3N5 or its primary gaseous decomposition products.

United States Patent Zubler 1 51 July 25, 1972 [54] GETTERING PROCESSESFOR References Cited ELECTRIC LAMPS AND SIMILAR UNITED STATES PATENTSDEVICES 1,565,579 12/1925 MacRae.. ..3l6/25 x [72] Inventor: Edward G.Zubler, Chagrin Falls, Ohio 1 1 4/1930 1,817,333 8/193! [73] Assignee:General Electric Company 3,475,072 10/1969 22 Filed: Dec. 28, 1970353L175 9/1970 [21} AppLN -i 1 Primary Examiner-John F. Campbell i DAssistant Examiner-Richard Bernard Lazarus Related App on anAttorney-Henry P. Truedell, Frank L. Neuhauser, Oscar B. [62] Divisionof Ser. No. 826,888, May 22, 1969, Pat. No. Waddell and Joseph Forman[57] ABSTRACT [52] U.S.Cl In the process of genering, the rate andextent (to about )0 [51] f 2 1 6 percent) of thermal decomposition of PN, getter are in- [58] held of Search l I creased by providing acatalyzer metal in contact with the solid RN, or its primary gaseousdecomposition products.

6 Claims, 3 Drawing Figures P'A'IENTEDmzs me 0 o o n 0 o 0 0 0 0 0 0 .00 0 0 0 E H m 9 8 m 2 6 2 4 2 2 2 0 2 M 6 I M M 0 I 6 6 4 2 0 TIME M/NUT55 lnven tor:

Edward G. ubL T by His A L-orneg GETTERING PROCESSES FOR ELECTRIC LAMPSAND SIMILAR DEVICES CROSS-REFERENCES TO RELATED APPLICATIONS Thisapplication is a division of application Ser. No. 826,888, filed May 22,1969 now US. Pat No. 3,580,856.

BACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates generally to the manufacture of electrical devices comprising asealed envelope containing an electric energy translation element suchas a filament or electrodes, and more particularly to an improvement inthe gettering of such devices to clean up residual deleterious gases,particularly when the getter is phosphorus nitride.

2. Description of the Prior Art For many years past it has beencustomary to effect the clean up of residual gases in electricincandescent lamps by means of phosphorus, which is customarily appliedas a slurry or suspension to the filament which is mounted in the lampenvelope, after which the envelope is evacuated and sealed, with orwithout an inert gas filling therein, and the getter is vaporized orsublimed by energizing the filament. In the case of gas-filled lamps itis the practice to use red phosphorus alone, whereas in vacuum lamps thered phosphorus is mixed with a material such as cryolite.

Under certain conditions, including high temperatures attained in thecourse of lamp sealing, and especially during periods of highatmospheric humidity, the red phosphorus is subject to deterioration. Ithas recently been found that high stability and superior getteringaction may be achieved by the use of pure crystalline phosphoruspentanitrade (P N as more fully disclosed and claimed in US. Pat. No.3,475,072, issued Oct. 28, 1969 to J. A. Graves, and assigned to thesame assignee as the present application.

SUMMARY OF THE INVENTION 1 have determined that the extent of thermaldecomposition of P N from an open container in vacuum to nitrogen and acondensate is limited to a range of about 40-50 molar percent attemperatures in excess of about 800 C.

l have discovered, in accordance with this invention, that the efficacyor degree of decomposition of phosphorus nitride at the same temperatureis materially enhanced, even to complete decomposition, by heating thephosphorus nitride while it, or its gaseous decomposition species, is inintimate physical contact with one of a number of metals which serve asa catalyzer for the decomposition process. This improved degree ofdecomposition occurs irrespective of the surrounding pressure. Apresently preferred catalyzer is tungsten, especially for use intungsten filament incandescent lamps. While tests have shown that theextent of decomposition is the same for amorphous and for crystalline PN the pure crystalline material is preferred.

BRIEF DESCRIPTION OF THE DRAWING In the drawing,

FIG. 1 is a front elevation of a conventional general purposeincandescent electric lamp in which the getter may be applied to thefilament and vaporized therefrom;

FIG. 2 is a fragmentary side elevation of an incandescent lamp having aseparate container from which the getter is vaporized; and

FIG. 3 is a graph showing decomposition plots of phosphorus nitridegetter with and without the presence of a catalyzer metal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The effect of catalyzers on P Nhas been clearly demonstrated in the laboratory by me where, forexample, a 0.200 to 0.400 mg sample was weighed in a small quartzcontainer which was then positioned in a quartz tube surrounded by asmall furnace. After the system was pumped down to 10' torr for about 3hours, the temperature of the sample was raised to 700 C. Afterapproximately 30 minutes at this temperature, the system was isolatedand the temperature raised to l,025 C in about nine minutes. The P Nstarts to decompose at about 800 C producing nitrogen and a non-volatilematerial which condenses in the cool zone. The nitrogen pressure in thecalibrated volume is continuously recorded with a thermistor gauge. Asmall mass spectrometer is used for qualitative analysis of the gasevolved. From the original weight of P N and the nitrogen pressure, thepercent decomposition can be calculated and plotted as a function oftime. Various batches of P N showed some differences in rate ofdecomposition; however, the various batches showed the same limiteddecomposition property in the range of about 40 to 50 percent. In FIG.3, the curve A is a typical and reproducible composition plot for aparticular batch.

The effect of a catalyzer in accordance with the invention was firstdemonstrated by adding about l5 mg of high purity tungsten powder, about1 micron particle size, to some 0.200 mg P N samples. The temperaturewas increased from 700 to 1,025 C at the rate shown by the curve T inFIG. 3 where the temperature is shown along the right-hand ordinate.

The tungsten powder had a dramatic effect on the rate and extent ofdecomposition as demonstrated by curve B in FIG. 3. The decompositionwas greatly increased'by the tungsten particles, curve B actuallyshowing decomposition slightly exceeding l00 percent which is within thelimits of experimental error. It will be evident that for maximumgettering efficacy in a lamp, this rapid and complete decomposition ofthe P -,N to some form of phosphorus is desirable. In a lamp, only asmall fraction of the getter is in physical contact with the usualtungsten filament, and it is therefore desirable to incorporate a smallquantity of tungsten powder into the slurry which may comprise P Ngetter in amyl acetate.

Referring to FIG. I, the slurry of P N getter and catalyzer is mostconveniently applied to the filament l of an incandescent lamp. Asherein illustrated, the lamp is of a conventional type comprising asealed glass bulb 2 containing the filament 1 which may be a helicalcoiled-coil having its ends secured to lead-in conductors 3 and 4 whichhave portions thereof hermetically sealed in the press or pinch portion5 of'a glass stem tube 6 which has a flared lower end (not shown) sealedby fusion to the neck of bulb 2. The lead wires 3 and 4 extend torespective shell 7 and eyelet 8 contacts of a screw base 9 which issecured to the lower end of the neck of bulb 2. The bulb is evacuatedand may be filled with a suitable inert gas through an exhaust tube 10which extends through stem tube 6 and communicates with the interior ofbulb 2 through an exhaust aperture 11. It is well recognized in the artthat such a conventional gas-filled lamp is manufactured and operateswith the gas filling pressure at substantially atmospheric pressure orsometimes in excess of atmospheric.

Since only a small fraction of the P N on the lamp filament l is inphysical contact with the tungsten surface and the temperature isincreased to a high value, about 1,900 C for example, in a very shorttime, the tungsten filament surface probably plays a very minor role inthe decomposition process. When the mixed powdered P N and tungstenpowder are flashed off the filament as small particles, they circulatein the gas stream past the hot filament l and are maintained in closephysical relationship to achieve complete decomposition of the P N Inone lamp test a relatively poor batch of P N based on previous lamptests, was used with the tungsten catalyst of the invention and gaveimproved results when subjected to testing for the presence of oxygen bythe extremely sensitive testing equipment described and claimed in US.Pat. No. 3,194,l 10 to H. .l. Eppig et al. The lamps of this and othertests were at a pressure approximately equal to atmospheric pressure(760 mm. Hg); however, the same results would be expected if the lampswere vacuum lamps or at a pressure in excess of atmospheric pressure.

In FIG. 2 there is illustrated an alternative method of heating thegetter to achieve complete decomposition of the P N In this case the P Npowder 12 is placed in a container 13 which may be a tubular cup orcylinder of suitable metal such as nickel or molybdenum, for example,and which may be supported by a wire member 14 secured at one end of thecontainer l3 and at its other end to a lead wire 4. The upper end of thecontainer 13 is filled with a filter-like mass or plug 15 of wire madeof the catalyzer metal, preferably tungsten. In this case, the cup 13 isheated by any suitable means such as high frequency induction to atemperature above the decomposition temperature (800 C) of the P N andthe tungsten wire ball 15 is in the path of, and comes in contact with,the gaseous decomposition products. Presumably, the tungsten wire 15catalyzes the further decomposition of gaseous P,N,, species producingthe more volatile P species.

In laboratory tests similar to those described above, but with the P Ncontained in a quartz cylinder or cup having a tungsten wire plug at thetop thereof, complete breakdown of the P N was achieved at a slightlylower rate than with the mixed powdered tungsten and P N The proportionof tungsten powder to P .,l J powder is not critical. Amounts oftungsten from 10 to percent by weight of the P N have given goodresults, and up to 100 percent, i.e., equal weights of tungsten and P Ndid not increase the yield nor did it adversely effect the light outputofthe lamp. By way of example only, a slurry of l N plus 20 weightpercent tungsten in butanol or amyl acetate may be applied to thefilament ofa 40 watt lamp.

A number of metals other than tungsten also enhance the rate anddecomposition of P N For example, molybdenum and nickel, like tungsten,increased the extent of decomposition to about 100 percent. Other metalsuseful as catalyzer are rhenium, titanium, zirconium, rhodium,palladium, osmium, iridium and platinum. Metals such as silver, aluminumand magnesium have too high a vapor pressure and would also adverselyeffect the tungsten filament of the lamp. It was also shown that puregraphite had no effect on the decomposition of P -,N,,.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In the process of gettering electrical devices having sealedenvelopes by the thermal decomposition of pure RN, in the envelope, theimprovement which comprises materially enhancing the extent ofdecomposition of the P N by heating it while in intimate physicalcontact with finely divided catalyzer metal selected from the groupconsisting of tungsten, molybdenum, rhenium, nickel, titanium,zirconium, hafnium, vanadium, niobium, tantalum, rhodium, palladium,osmium, iridium and platinum.

2. The process defined in claim 1 wherein the catalyzer metal istungsten.

3. The process defined in claim 1 wherein the electrical device is anincandescent lamp and the mixture of P N and finely divided catalyzermetal is applied as a coating on the filament which is heated by passageof current therethrough to heat the coating to a temperature sufficientto decompose the P N 4. The process defined in claim 3 wherein thecatalyzer metal is tungsten.

5. In the process of gettering electrical devices having sealedenvelopes by the thermal decomposition of pure P N in the envelope, theimprovement which comprises materially enhancing the extent ofdecomposition of the P N by directing its gaseous decomposition productsthrough a filter-like mass of fine wire of a catalyzer metal selectedfrom the group consisting of tungsten, molybdenum, rhenium, nickel,titanium, zirconium, hafnium, vanadium, niobium, tantalum, rhodium,palladium, osmium, iridium and platinum.

6. The process defined in claim 5 wherein the catalyzer metal istungsten.

2. The process defined in claim 1 wherein the catalyzer metal istungsten.
 3. The process defined in claim 1 wherein the electricaldevice is an incandescent lamp and the mixture of P3N5 and finelydivided catalyzer metal is applied as a coating on the filament which isheated by passage of current therethrough to heat the coating to atemperature sufficient to decompose the P3N5.
 4. The process defined inclaim 3 wherein the catalyzer metal is tungsten.
 5. In the process ofgettering electrical devices having sealed envelopes by the thermaldecomposition of pure P3N5 in the envelope, the improvement whichcomprises materially enhancing the extent of decomposition of the P3N5by directing its gaseous decomposition products through a filter-likemass of fine wire of a catalyzer metal selected from the groupconsisting of tungsten, molybdenum, rhenium, nickel, titanium,zirconium, hafnium, vanadium, niobium, tantalum, rhodium, palladium,osmium, iridium and platinum.
 6. The process defined in claim 5 whereinthe catalyzer metal is tungsten.